Review Question - QID 2029

Figure C is a histologic slide of a unicameral bone cyst, which is most likely to present with a pathological fracture in a child.

A unicameral bone cyst is a non-neoplastic, serous fluid-filled bone lesion that is thought to result from temporary failure of medullary bone formation near the physis. It is usually found in patients less than 20 years of age. Common locations are the proximal humerus, proximal femur, distal tibia, ilium, calcaneus, distal radial, and occasionally the metacarpals and phalanges. Patients are usually asymptomatic and may present with pathologic fractures in up to 50% of cases.

Wilkins performed a review of unicameral bone cysts. He reports that radiographs of fractures usually reveal a non-displaced or minimally displaced fracture through an area of thin cortical bone. Occasionally a fragment of the cyst wall will fracture and fall into the fluid-filled cavity. This is known as the "fallen leaf sign."

Sung et al. performed a review of 187 patients younger than 20 treated surgically for humeral or femoral unicameral bone cysts with either injection of corticosteroids, curettage plus bone grafting, or a combination of steroids, demineralized bone matrix, and bone marrow aspirate (SDB). They conclude that SDB is a reasonable first treatment for unicameral bone cysts in the humerus and femur in patients younger than 20 years, being less invasive yet comparable to curettage in preventing recurrence.

Figure A is a histologic slide demonstrating cavernous, blood-filled spaces without an endothelial lining, characteristic of an aneurysmal bone cyst. Figure B is a histology slide demonstrating atypical malignant cells in a background of malignant osteoid, characteristic of osteosarcoma. Figure C is a histology slide demonstrating a cyst with a thin layer of connective tissue and hemosiderin, characteristic of a unicameral bone cyst. Figure D is a histology slide demonstrating sheets of monotonous small, round blue cells with prominent nuclei and minimal cytoplasm, characteristic of Ewing's sarcoma. Figure E is a histologic slide demonstrating numerous giant cells, characteristic of giant cell tumor of bone. Illustration A is a radiograph of a pathological proximal humerus fracture through a unicameral bone cyst with an arrow indicating the fallen leaf sign.

Incorrect Answers:
Answer 1: Figure A is a histologic slide representative of an aneurysmal bone cyst.
Answer 2: Figure B is a histologic slide representative of osteosarcoma.
Answer 4: Figure D is a histologic slide representative of Ewing's sarcoma.
Answer 5: Figure E is a histologic slide representative of giant cell tumor of bone.